PROJECT SUMMARY/ABSTRACT Activating mutations in RAS or mutations in RAS regulatory genes that lead to hyperactivated RAS are drivers of human cancer. Neurofibromatosis type 1 (NF1) is a developmental disease caused by loss of the NF1 tumor suppressor gene, which encodes neurofibromin, a large protein that inactivates RAS through its GTPase activating function. Loss of NF1 leads to the hyperactivation of RAS and increases the risk of developing multiple cancers, including malignant peripheral nerve sheath tumors (MPNSTs). RAS activation promotes signaling of several downstream pathways that drive oncogenesis, including the PI3K and MAPK signaling pathways. Although many attempts have been made to target RAS either directly or indirectly, none of them have yet been successful in the clinic and therefore new insights into the precise mechanisms that govern RAS signaling are needed. The small GTPase protein ARF6 has been implicated in the growth and metastasis of many cancers. In uveal and cutaneous melanomas, ARF6 promotes the trafficking of oncogenic G?q or ?-catenin to appropriate intracellular locations where signaling or transcription is enhanced. Our preliminary data suggest that ARF6 may also be regulating RAS signaling in NF1-deficient MPNSTs by controlling RAS intracellular trafficking and that ARF6 controls proliferation of NF1-deficient tumor cells. Activated ARF6 appears to be necessary for RAS lipid modifications, such as palmitoylation and farnesylation, which are essential for RAS membrane localization and for binding with its effectors, RAF and p110 (a PI3K catalytic subunit). Based on these preliminary data, we hypothesize that ARF6 promotes tumorigenesis in NF1-deficient tissues by controlling RAS trafficking and signaling. We will test this hypothesis by pursuing the following aims. In Aim 1, we will determine whether loss of NF1 activates ARF6 to control tumor cell proliferation by regulating RAS subcellular localization and signaling. These studies will involve multidisciplinary approaches to determine how NF1 regulates ARF6 and how ARF6 controls cell proliferation, RAS lipid modifications, intracellular localization, and signaling. In Aim 2, we will assess ARF6 function in tumor growth in an orthotopic xenograft mouse model of NF1-deficient human MPNST. These studies will allow us to determine how ARF6 knockdown or pharmacologic inhibition affects in vivo growth of tumors derived from human cancer cells in an immunocompromised mouse model. In Aim 3, we will determine whether ARF6 is necessary for tumor formation and growth in a genetically engineered mouse model of NF1- deficient cancers. These studies will involve either knocking out Arf6 or pharmacologically inhibiting ARF6 in immunocompetent Nf1+/-; Trp53+/- mice to determine the in vivo role of ARF6 in NF1-deficient tumorigenesis in the presence of a competent immune system. This proposal will provide essential training in multidisciplinary approaches and expand the candidate?s knowledge of cancer biology (K99 phase) and will establish the foundations for an independent research program focused on the roles of ARF6 in cancers driven by not only NF1 deficiency but also hyperactivated RAS (R00 phase).